Re: [RFC PATCH v2 1/6] KVM: gmem: Truncate pages on punch hole

[Sean Christopherson]

On Thu, Sep 21, 2023, isaku.yamahata@xxxxxxxxx wrote:
> From: Isaku Yamahata <isaku.yamahata@xxxxxxxxx>
> 
> Although kvm_gmem_punch_hole() keeps all pages in mapping on punching hole,
> it's common expectation that pages are truncated.  Truncate pages on
> punching hole.  As page contents can be encrypted, avoid zeroing partial
> folio by refusing partial punch hole.
> 
> Signed-off-by: Isaku Yamahata <isaku.yamahata@xxxxxxxxx>
> ---
>  virt/kvm/guest_mem.c | 14 ++++++++++++--
>  1 file changed, 12 insertions(+), 2 deletions(-)
> 
> diff --git a/virt/kvm/guest_mem.c b/virt/kvm/guest_mem.c
> index a819367434e9..01fb4ca861d0 100644
> --- a/virt/kvm/guest_mem.c
> +++ b/virt/kvm/guest_mem.c
> @@ -130,22 +130,32 @@ static void kvm_gmem_invalidate_end(struct kvm_gmem *gmem, pgoff_t start,
>  static long kvm_gmem_punch_hole(struct inode *inode, loff_t offset, loff_t len)
>  {
>  	struct list_head *gmem_list = &inode->i_mapping->private_list;
> +	struct address_space *mapping  = inode->i_mapping;
>  	pgoff_t start = offset >> PAGE_SHIFT;
>  	pgoff_t end = (offset + len) >> PAGE_SHIFT;
>  	struct kvm_gmem *gmem;
>  
> +	/*
> +	 * punch hole may result in zeroing partial area.  As pages can be
> +	 * encrypted, prohibit zeroing partial area.
> +	 */
> +	if (offset & ~PAGE_MASK || len & ~PAGE_MASK)
> +		return -EINVAL;

This should be unnecessary, kvm_gmem_fallocate() does

	if (!PAGE_ALIGNED(offset) || !PAGE_ALIGNED(len))
		return -EINVAL;

before invoking kvm_gmem_punch_hole().  If that's not working, i.e. your test
fails, then that code needs to be fixed.  I'll run your test to double-check,
but AFAICT this is unnecesary.

> +
>  	/*
>  	 * Bindings must stable across invalidation to ensure the start+end
>  	 * are balanced.
>  	 */
> -	filemap_invalidate_lock(inode->i_mapping);
> +	filemap_invalidate_lock(mapping);
>  
>  	list_for_each_entry(gmem, gmem_list, entry) {
>  		kvm_gmem_invalidate_begin(gmem, start, end);
>  		kvm_gmem_invalidate_end(gmem, start, end);
>  	}
>  
> -	filemap_invalidate_unlock(inode->i_mapping);
> +	truncate_inode_pages_range(mapping, offset, offset + len - 1);

The truncate needs to happen between begin() and end(), otherwise KVM can create
mappings to the memory between end() and truncate().

> +
> +	filemap_invalidate_unlock(mapping);
>  
>  	return 0;
>  }
> -- 
> 2.25.1
>